The object of the present invention is a process for treating, by means of a catalytic solution, a gas under pressure containing at least hydrogen sulfide wherein the sulfur recovery stage can be carried out under high pressure and the catalytic solution is regenerated with an oxygen-containing gas, and a device allowing dispersion of this gas into fine droplets in the solution.
The process according to the invention is notably applied to air regeneration of the catalytic solution during a  less than  less than redox greater than  greater than  process for desulfurizing a gas containing at least hydrogen sulfide. This process uses a catalytic solution comprising at least one polyvalent metal chelated by at least one chelating agent under suitable conditions for oxidation of the hydrogen sulfide to elementary sulfur and simultaneous reduction of the chelated polyvalent metal from a higher oxidation level to a lower oxidation level. A gaseous effluent practically free of hydrogen sulfide is recovered on the one hand and a catalytic solution at least partly reduced and containing elementary sulfur is recovered on the other hand. The solid elementary sulfur is separated from the partly reduced catalytic solution. At least part of said partly reduced catalytic solution freed from most of the solid elementary sulfur can be at least partly expanded. Said solution, at least partly expanded, is air-regenerated by natural entrapment of air into a regeneration zone consisting of at least one ejector associated with at least one gas/liquid separator.
The prior art describes many  less than  less than redox greater than  greater than  processes allowing to eliminate hydrogen sulfide. These processes however have several drawbacks.
In general, it is well-known to contact the partly reduced catalytic solution containing the solid elementary sulfur with air at atmospheric pressure in order to regenerate said solution and to recover the sulfur by flotation and/or decantation. When the gas to be desulfurized is under high pressure, preliminary expansion of the reduced catalytic solution containing the solid sulfur is therefore generally performed. This expansion leads to degassing of the reduced catalytic solution which generally causes foaming and clogging problems (due to the simultaneous presence of solid sulfur and dissolved gas) that can lead to plant stops.
The separation stage being in principle carried out after the expansion stage, the expansion stage thus involves all of the reduced catalytic solution. A large quantity of energy is therefore necessary to recompress all of the solution after the regeneration stage.
Air regeneration of the reduced catalytic solution is generally performed by air injection by means of a diffuser, at the bottom of a reactor containing said solution to be regenerated (air blower), as described for example in French patent application number 97/15,520 filed by the applicant. However, this procedure is not optimal. In fact, such a reactor is generally large in size in relation to the other apparatuses of the device because the regeneration rate of the catalytic solution is most often widely controlled by transfer of the oxygen of air to the liquid, therefore by the gas/liquid surface. Furthermore, air compressors can require a large amount of energy.
The prior art is furthermore described in patents U.S. Pat. No. 5,753,189, WO-96/29,289 A, DE-3,444,252 A, U.S. Pat. No. 4,859,436 and U.S. Pat. No. 4,532,118. However, none of these documents describes or suggests, alone or in combination, a process for desulfurizing a feed containing at least hydrogen sulfide wherein at least part of the reduced catalytic solution from stage b) of separation of the elementary sulfur from the reduced catalytic solution is expanded so as to degas the solution to be regenerated until a pressure allowing autonomous operation of the driving device intended for natural entrainment of air into the regenerator is reached. This device is an ejector.
The process according to the invention proposes a new approach for sulfur recovery and regeneration of the catalytic solution which notably affords the following advantages:
it prevents foaming and clogging problems due to the simultaneous presence of solid sulfur and dissolved gas in the reduced catalytic solution,
it minimizes the energy consumption required for recompression of the regenerated catalytic solution,
it minimizes the size of the catalytic solution regeneration zone and therefore reduces energy consumption in this zone.
The present invention relates to a process intended for desulfurization of a gaseous feed containing at least hydrogen sulfide, comprising at least the following stages:
a) contacting the gaseous feed consisting at least of hydrogen sulfide with a catalytic solution comprising at least one polyvalent metal chelated by at least one chelating agent, under suitable conditions for oxidation of the hydrogen sulfide to elementary sulfur and concomitant reduction of the chelated polyvalent metal from a higher oxidation level to a lower oxidation level (absorption stage), and recovering on the one hand a gaseous effluent substantially freed from hydrogen sulfide and, on the other hand, said catalytic solution at least partly reduced and containing elementary sulfur,
b) separating the solid elementary sulfur from the reduced catalytic solution,
c) regenerating the reduced catalytic solution in a regeneration zone,
d) recycling at least part of the regenerated catalytic solution to a stage of contacting the regenerated solution with a gaseous feed consisting at least of hydrogen sulfide.
It is characterized in that at least part of the reduced catalytic solution freed from most of the elementary sulfur and coming from stage b) is expanded, the gases produced upon expansion are discharged and stage c) of regeneration of the expanded reduced catalytic solution is carried out by contacting the circulating catalytic solution with a gas containing oxygen by means of at least one ejector in said regeneration zone.
Regeneration stage c) is then carried out by contacting the circulating catalytic solution with a gas containing oxygen, under suitable conditions allowing dispersion of the gas into very fine bubbles in the catalytic solution by increasing the velocity of circulation of said solution.
The ejector can be used under such conditions that the ratio of regeneration gas to liquid advantageously ranges between 0.1 and 10, preferably between 0.5 and 5.
The gaseous feed can be at a pressure ranging between 0.1 and 22 MPa in relative pressure and preferably between 4 and 10 MPa.
Said reduced catalytic solution depleted in elementary sulfur, obtained after stage b), can be fractionated into a major fraction F1 and a minor fraction F2, and the non-expanded major fraction F1 is recycled to absorption stage a).
The potential of said partly reduced catalytic solution is for example measured before the fractionation stage and fractions F1 and F2 are determined, the quantity of said fractions is controlled so as to maintain the ratio of the ferric ions to the ferrous ions substantially equal to 20.
Separation stage b) can be carried out at a pressure ranging between 0.1 and 20 MPa.
A gas/liquid separation stage is for example carried out after the catalytic solution regeneration stage in order to remove the excess gas in the regeneration zone.
An aqueous solution can be used as the catalytic solution in stage a).
The aqueous catalytic solution is for example a solution of a chelated polyvalent metal such as an aqueous solution of chelated iron produced for example from ferrous or ferric iron such as iron and ammonium or potassium sulfates, nitrates, thiosulfate, chloride, acetate, oxalate, phosphates, soluble salts such as ferrous iron and ammonium sulfate, ammonium ferric oxalate, potassium ferric oxalate.
Chelating agents are for example used alone or in admixture, such as organic compounds known for their complexing properties, for example acetylacetone, citric acid, salicylic acid, sulfosalicylic acid, tiron (catechodisulfonic acid), dimercapto-2-3 propanol and aminoacids such as EDTA (ethylenediamine tetraacetic acid), HEDTA (hydroxy2ethylenediamine triacetic acid), NTA (nitrilotriacetic acid), DCTA (diamino-1-2 cyclohexane tetraacetic acid), DPTA (diethylenetriamine pentaacetic acid), IDA (imonodiacetic acid).
The catalytic solution is for example an organic solution.
It is possible to use for example an organic solution consisting (i) of a solvent selected from the following products: N-methyl pyrrolidine, N-formylmorpholine, morpholine, dimethylsulfoxide, sulfolane, dimethylformamide, propylene carbonate, 1,4-dioxane, 4-hydroxy-4-methyl-2-pentanone, propylene glycol methyl ether, 2-butoxyethanol, 4-methyl-2-pentanone, 2,4-pentanedione, alone or in admixture, and (ii) of a chelated polyvalent metal of formula ML3 where L is of R1COCH2COR2 form with R1 and R2 selected from groups CH3, C2H5, C6H5CF3, C4H3S, and M is a polyvalent existing in at least two oxidation states.
According to a variant, the sulfur coming from separation stage b) is washed with water in order to recover the catalytic solution.
The invention also relates to a device intended for desulfurization of a gaseous feed containing at least hydrogen sulfide, said device comprising at least one zone for contacting said gaseous feed with a catalytic solution containing at least one polyvalent metal chelated by at least one chelating agent, under suitable conditions for oxidation of the hydrogen sulfide to elementary sulfur and concomitant reduction of the chelated polyvalent metal from a higher oxidation level to a lower oxidation level, a device intended for separation of the solid elementary sulfur from said reduced catalytic solution, a regeneration zone for said reduced catalytic solution, said regeneration zone comprising suction means intended for a regeneration gas containing oxygen.
It is characterized in that it comprises at least one means (11) intended for expansion of said solution, located after separation device (2), a means intended for separation of the gases produced and at least one ejector (16) arranged in the regeneration zone, suited to obtain dispersion of the regeneration gas into very fine bubbles in said solution.
The ejector can be connected to a gas/liquid separation device.
The contacting zone comprises for example at least one contactor/reactor selected from the following list: reactor with stacked or random packing, static mixer, turbulent-jet impactor, hydro-ejector, atomizer, wire contactor, bubble column.
The process and the device can be applied to desulfurization of a natural gas.